Other footage, still unreleased, shows that the two suspects stayed at the scene to watch the carnage unfold, a federal law enforcement official with knowledge of the investigation told CNN's Susan Candiotti.

"When the bombs blow up, when most people are running away and victims were lying on the ground, the two suspects walk away pretty casually," said the official, who has seen the unreleased video. "They acted differently than everyone else."

While video of at least one suspect planting the bomb exists, the FBI had chosen not to release it, according to the official. One reason, according to the official, is that were the media to repeatedly show the suspects leaving the bomb, it might cause some people to overreact if they came into contact with them.

DesLauriers said intelligence had been developed on the first suspect "within the last day or so." The official who spoke with CNN said images of the second suspect were isolated Wednesday.

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